Coating of substrates by means of CVD (chemical vapour deposition) is widely used. For example, diamond coatings or films produced by means of CVD are suitable as protective coatings for tools and precision components, e.g. seals, and for use in micro electro mechanical systems (MEMS), in field emission displays (FED) and in apparatuses that exploit surface acoustic waves (SAW).
The use of CVD coating methods for diamond coating on an industrial scale requires safe-to-handle process parameters, uniform coatability of, e.g., highly curved or pointed surface elements and edges, and coating equipment of adequate size. These requirements are not met by all CVD coating methods described so far.
For example, the microwave plasma coating method described in U.S. Pat. No. 5,849,079 and U.S. Pat. No. 5,770,2760 requires an overall pressure of 5×103–105 Pa, which is relatively high. As the cell volume of microwave reactors is very small, the coatable area per coating operation is a maximum of roughly 200 cm2, which leads to very high coating costs. In microwave plasma coating methods, larger layer thicknesses are additionally deposited on edges and tips, so that uniform coating is not possible.
These disadvantages are avoided in the method described in EP 0724026 and WO 01/04379, in which the plasma is generated by a discharge, preferably by a high arc current discharge. This process proceeds preferably at an overall pressure that is lower than with the microwave plasma process by a factor of roughly 103 and enables a coatable surface per coating operation that is higher by a factor of 101–102 or more. Furthermore, the method described in EP 0724026 and WO 01/04379 permits uniform coating of highly curved surfaces and edges and tips.
Coatings deposited by means of CVD methods are described as microcrystalline or nano-crystalline with respect to the average size of the deposited crystallite particles, which may be determined, e.g., by transmission electron microscopy. With microcrystalline coatings, the average crystallite size is greater than 1 μm and varies typically between 1 and 10 μm. With nanocrystalline coatings, the average crystallite size, by contrast, is smaller than 1 μm and especially smaller than 100 nm. Control and adjustability of the average crystallite size are important for optimizing properties of the diamond coatings, such as their hardness, surface roughness and their electrical and optical characteristics.
Whereas the microwave plasma techniques permit the deposition of nanocrystalline diamond coatings (U.S. Pat. No. 5,772,760 and Ann. Rev. Mater. Sci. 29 (1999), 211–259), this has not yet been reported for CVD coatings techniques that can be used on an industrial scale.
The present invention was therefore directed to finding an industrial scale, i.e. economically viable, CVD method for producing diamond coatings that does not have the disadvantages described in the prior art, or only to a lesser extent, and permits especially the deposition of nanocrystalline diamond coatings. Further objects of the present invention result from the following detailed description.